releasenotes: add a file in which to record release notes

Authored by martinvonz.

Description

releasenotes: add a file in which to record release notes

I've just spent a few very boring hours going through the changelog
for the 5.0 release (829 commits). We only had 5 commits that used the
syntax that the release notes extension expects. This commit adds a
file in which we can record important changes. The file should
preferably be edited in the patch that makes the important change, but
it can also be edited after (I think this is an important benefit
compared to the release notes extension).

I'm thinking that we can rename the file from "next" to "5.1" or
something when it's time, and then we'd create a new "next" file on
the default branch.

I've used the syntax that we use on the our wiki in the template, but
I don't care much that we use any valid syntax at all. The idea is
mostly to record important changes when they happen. I expect that
some copy editing will be needed at release time anyway.

Differential Revision: https://phab.mercurial-scm.org/D6332